United States Patent [I91

Flytani-Stephanopoulos et al.

[54] HIGH TEMPERATURE REGENERATIVE

[75] Inventors: Maria Flytani-Stephnnopoulos, Pasadena; George R. Gavalas, Altadena, both of Calif.; Satish S. Tamhankar, Scotch Plains, N.J.

1731 Assignee: California Institute of Technology, Pasadena, Calif.

[21] Appl. No.: 717,333 [22] Filed: Mar. 29,1985

[51] Int. Cl.4 ......................... COlF 7/02; COlG 49/02 [52] U.S. c1. .................................... 423/593; 423/594;

423/599; 423/600; 423/604,423/606; 423/622; 423/625; 423/632; 502/400; 502/406; 502/415

[58] Field of Search ............... 502/406, 415, 502, 517, 502/307,316,318,308,323,353,354, 324,331, 346, 524, 525; 423/210.5, 213.5, 236, 231, 593,

594, 604, 606, 622, 632, 599, 600

H2S SORBENTS

[561 References Cited U.S. PATENT DOCUMENTS

3,124,452 3/1964 Kraft ................................... 75/65 R 3,403,112 9/1968 Sze et al. ............................. 502/316 3,417,145 12/1968 Nemec et al. ....................... 502/318 3,526,675 9/1970 Croce et al. ........................ 502/324 3,567,793 3/1971 Colling et al. ...................... 502/324 3,686,347 SA972 Dean et al. .......................... 502/324 3,716,497 2/1973 Courty ................................ 502/316 3,855,388 12/1974 Rosinski .............................. 502/318 3,925,259 12/1975 Kane ................................... 502/318 3,954,938 5/1976 Meissner .......................... 423/210.5 4,069,171 1/1978 Lemkey et al. ..................... 502/323 4,086,264 4/1978 Brooks et al. ....................... 502/318 4,089,809 5/1978 Farrior, Jr. ......................... 502/406 4,197,277 4/1980 Sugier et al. ................... 423/573 G 4,207,209 6/1980 Matsuda et al. .................... 502/406 4,293,444 10/1981 Parthasarathy ..................... 502/307 4,313,820 2/1982 Farha, Jr. et al. .................. 423/230 4,442,078 4/1984 Jalan et at. .......................... 423/230

[i l l Patent Number: 4,729,889 [45] Date of Patent: Mar. 8, 1988

4,455,286 6/1984 Young et al. ....................... 423/230 4,470,958 9/1984 Van Gelder et al. ............ 423/210.5 4,478,800 10/1984 Van der Wal et al. ............. 423/231 4,489,047 12/1984 de Jong et al. ..................... 502/517 4,519,992 5/1985 Alkhazov et al. .................. 423/231 4,539,101 9/1985 Oleck et al. .................... 208/251 H

OTHER PUBLICATIONS Marcilly, C. et al, "Preparation of Highly Dispersed Mixed Oxides", J. Am. Ceramic SOC., 53, No. 1, 56 (1970). Primaly Examiner-Helen M. S. Sneed Assistant Examiner-Anthony McFarlane Attorney, Agent, or Firm-Marvin E. Jacobs

[571 ABSTRACT Efficient, regenerable sorbents for removal of HIS from high temperature gas streams comprise porous, high surface area particles. A first class of sorbents comprise a thin film of binary oxides that form a eutectic at the temperature of the gas stream coated onto a porous, high surface area refractory support. The binary oxides are a mixture of a Group VB or VIB metal oxide with a Group IB, IIB or VI11 metal oxide such as a film of V-Zn-0, V-Cu-0, Cu-Mo-0, Zn-Mo-0 or Fe-Mo-0 coated on an alumina support. A second class of sor- bents consist of particles of unsupported mixed oxides in the form of highly dispersed solid solutions of solid compounds characterized by small crystallite size, high porosity and relatively high surface area. The mixed oxide sorbents contain one Group IB, IIB or VIIB metal oxide such as copper, zinc or manganese and one or more oxides of Groups IIIA, VIB or VI1 such as aluminum, iron or molybdenum. The presence of iron or aluminum maintains the Group IB, IIB or VIIB metal in its oxidized state. Presence of molybdenum results in eutectic formation at sulfidation temperature and improves the efficiency of the sorbent.

10 Claims, 6 Drawing Figures

https://ntrs.nasa.gov/search.jsp?R=20080004085 2020-03-31T16:10:45+00:00Z

US. Patent Mar. 8,1988

I I I I I I I

Sheet 1 of 4 4,729,889

t

Fig. /.

800 -

700 -

c E 600- n a Y

v) 500- N I I- 400- w -I

5 300- 0

200 -

CAT - 3 N

P = 1 atm t c-3 -I T s = 7 0 0 ° C I N L E T H 2 S = 4 0 0 0 p p m S.V .= 2806 hr'l IH203 = 0 -

C = CYCLE NO.

TREGN = 700 OC -

-

-

-

100- -

NORMALIZED ABSORPTION TIME ( t / t * )

Fig. 2.

US. Patent Mar. 8,1988 Sheet 2 of 4 4,729,889

- 600- k 0.

v) 500- N I t; 400- -1 I- D 300- 0

900

TS= 7OOOC INLET H2S = 2000 ppm

S. V. = 2727 hr-1 H 2 0 / H 2 S = 2911 (CYCLES 3 & 4 )

TREGN = 7OOOC

:::L 8.0

I . i I C I - - - - I I I

0.2 0.4 0.6 0.8 1.0 NORMALIZED ABSORPTION TIME (t / t*)

Fig. 3. 280 I I I I I I I I I I

240

c

200

v)N 160 I

P - L 120 -I I- 3 0 80

SORBENT CFA (2CuO. F e 2 0 3 .AI 203) c-4 - P - l o t m

T s = 65OOC H ~ S = I . O V O I ~ / O , H 2 = 2 0 v o l o / o

- INLET{ H 2 0 ' 25 v O I O/o S.V.= 2210hr- l

C = C Y C L E NO. TRE GEN = 6 5 0 ° C -

NORMALIZED ABSORPTION TIME (t / t*)

Fig. 5.

US. Patent MU. 8,1988 Sheet 3 of 4 4,729,889

2 4 0

2 0 0 - E P P

v)

I c -I I- 3 0

I 6 0 Y

N

w 120-

80

40

280 I 1 I I I I

CM-2 (CuMoO4: Mo03/ALUMINA) -

-

-

-

-

H20/H25= 25/1 S. V. = 2060 hr - 1 TREGEN = 65OOC (65OOC) (65OOC (WITH N2/AIR=70/30)

C=CYCLE NO. I I I I I I I I I I 11 I I I I I

I I I I

X

1.0 1.2

NORMALIZED ABSORPTION TIME ( t / t *)

Fig. 4.

US. Patent Mar. 8,1988 Sheet 4 of 4 4,729,889

160-

1 8 0 J SORBENT C M - 5 (BULK C U M O O ~ * A I ~ 0 3 )

Ts'538OC (C- l AND C - 2 ) P - l a t r n

65OOC (C-3 ) H 2 S = 5 0 0 0 p p m , H2=15vol Oi0

NORMALIZED ABSORPTION TIME ( t / t * )

Fig. 6.

4,729,889 1 2

Kuney, D. K., Belyaevskaya, L. V., and Zelikman, A. N., Russian J. of Inorganic Chemistry 11. 1063 (1966)

11. Longo, J. M., Horowitz, H. S. and Clavenna, L. R., “A Low Temperature Route to Complex Oxides.” Advances in Chemistry Series No. 186, p. 139, Amer- ican Chemical Society, Washington, D.C., 1980.

12. Singh, B. N., Banerjee, R. K., and Arora, B. R., J. Them Analysis 18, (1980).

13. ~ ~ i t a , G. A., El-Tawil, s. z., brahim, A. A. and Felix, N. s., Themwhim. Acta 36, 359 (1980).

14. Marcilly, C., Courty, P. and Delmon, B., J.Am. Ceramic Soc. 53, No. 1, 56 (1970).

15. U.S. Pat. No. 1,851,312, W. J. Huff. 16. U.S. Pat. No. 2,019,468, T. S. Bacon.

18. U.S. Pat. NO. 3,429,656, W. F. Taylor et al. 19. U.S. Pat. No. 3,739,550, b e l Martin et al. 20. U.S. Pat. No. 4,197,277, Andre Sugier et al.

HIGH TEMPERATURE REGENERATIVE HzS SORBENTS

ORIGIN O F THE INVENTION The invention described herein was made in the per-

formance of work under a NASA contract and is sub- ject to the provisions of Section 305 of the National Aeronautics and Space Act Of 1958, Public h W 83-568 10 (72 Stat. 435: 42 USC 2457).

5

BACKGROUND O F THE INVENTION The present invention relates to novel sorbents for

high temperature regenerative removal of HzS from gas 15 17. U.S. Pat. No. 2,551,905, S. P. Robinson. streams and, more particularly, gas streams resulting from the gasification of coal and heavy oil residues.

High temperature desulfurization Of coal-derived 21. U.S. Pat. No. 4,207,209, Sinpei Matsuda et al. gas Offers potential improvements On the thermal effi- 20 22. U.S. Pat. No. 4,283,380, Robert Voirin et al. ciency of systems using coal gasification such as power Plants (high temperawe fuel Cells, combined Cycle) and synthesis gas conversion plants (ammonk methanol). Over the last ten years, several sorbents have been pro- posed and investigated for the regenerative removal of 25

the main sulfur compound, Le., hydrogen sulfide, from Discussion of the Prior Art fuel gas at high temperatures. The level of H2S removal The thermodynamics of various sorbents have been needed depends on the end use of the fuel gas. For analysed in (1) and (2) among other reports. Compre- power plant combustion purposes, removal down to 30 hensive surveys of experimental work encompassing about 100 ppm is adequate, but for molten carbonate various high temperature sorbents have also been pub- fuel cell applications removal down to a level of 1 ppm lished (1-7). may be required. The overall performance of a sorbent depends on a

List of Prior References 35 variety of properties. Thermodynamics and kinetics of 1. q-hemistry of Hot G~ cleanup in coal Gasification sulfidation are obvious factors, for they determine the

and Combustion,” MERC Hot Gas Cleanup Task overall sulfur capacity before breakthrough of some Force. predetermined determined level of Hz. Kinetics encom- MERC/SP-78/2, Feb. 1978. passes the rates of purely chemical steps as well as the

2. “Studies Involving High Temperature Desulfuriza- 40 rate of pore diffusion and, more crucially, diffusion in tionmegeneration Reactions of Metal Oxides for the the sulfide product layer. Surface area and pore size Fuel cell program, ” Final Report to DOE, Contract distribution are very important sorbent properties as No. 31-109-38-58049 by Gmer, InC.9 w d t w * Mass., they determine the rate of these diffusional processes. Feb. 1981. 45 Zinc oxide, one of the most promising and wide1 y stud-

3- Westmoreland, p. R-3 Gibson, J. B.9 and Harrison, D- ied sorbents, has very high equilibrium constant for sulfidation but in its unsupported form it suffers from P., Environ. Sci. Technol. 11, 488 (1977).

4. Westmoreland, P. R. and Harrison, D. P., Environ. slow kinetics limiting its sulfidation capacity. Iron ox- Sci. Technol. 10, 659 (1976). 5. Grindley, T. and Steinfeld, G., u D ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ and 5o ide, on the other hand, has rapid kinetics but its equilib-

Testing bf Regenerable Hot coal G~ Desulf-- rium constant for sulfidation is not adequate for the tion sorbents, 9’ METC, DOE/MC/16545-1125, act. degree of H2S removal required in the molten carbon- 1981. ate fuel cell application.

6. Anderson, G. L. and Garrigan, P. C., “Gas Process- The other important sorbent properties refer to stabil- ing Technology for Integrating Coal Gasifiers with 55 ity or regenerability in extended use, the operating con- Molten Carbonate Fuel Cells.” Paper presented at the ditions required for regeneration, and the composition Electrochemical Society Meeting, Montreal, Quebec, of the regeneration off-gas, which largely determines Canada, May 10-12, 1982. the choice of a downstream sulfur recovery process.

7. Stegen, G. E., “Development of a Solid Absorption using zinc it is well known process for Of from Gas*” Final that evaporative loss of metallic zinc places an upper Report DE-AC21-79ET1 1028 Battelle’ pacific limit on the sulfidation temperature. Loss of surface Northwest Laboratories, Mar. 1982.

8. Pollard, A. J., 6sProposed Phase Diagram for the area during regeneration places a lower limit on regen- System Vanadium pentoxide-Zinc Oxide.” U.S. eration temperature or necessitates a more complicated ~~~d Research Lab,, NRL R ~ ~ . 5960, Washington, 65 regenerative treatment. The regeneration off-gas, in- D.C., July 1963. cluding sulfur dioxide, hydrogen sulfide and elemental

sulfur, requires further treatment for sulfur recovery. Sci. Torino 95, 15 (1961). When the yield of elemental sufur is sufficiently high,

23. U.S. Pat. NO. 4,310,497, Andre Deschamps et al. 24. U.S. Pat. No. 4,455,286, J. E. Young et al. 25. Barrin, I. and Knacke, O., “Thermochemical Prop-

erties of Inorganic Substances,” Springer Verlag, 1973.

as an example

9. Cirilli, V., Burdese, A., and Brisi, C., Atti. &cad.

4,729,889 3 4

sulfur recovery can be simplified with significant over- all cost benefits (6).

These references emphasize the med for improved ~ ~ r b m t s fop the high temperature desulfurization of coalderived gas streams

the invention. The sorbents exhibit improved chemical stability in avoiding metal loss and undesirable reduc- tion at high temperatures and improved retention of pore volume and resistance to pore plugging whether supported or unsupported. Rapid kinetics of absorption

hl the U.S. patents listed above young et al (24) is the and good sorbent regenerability are achieved during in fie Of and operation at 5m-700" C. Rapid absorption rates are

mixtures as high temperature (500"-700" C.) regenera- b k sorbent for H2S in a hot gas stream fuel for a fuel 10 associated with cell. These mixtwes are not molten during m y stage of the dlesdhkation process and do not provide effective

capacity or regenerabilitye R o b d n (17) also discloses desulfurization of a gas at high temperature.

achieved by or minimizing the resistance

The sorbents of the invention remove H2S from hot gas streams to levels well below the target levels. The high Sulfur Capacity, lhkd surface area loss, high

state

1m" F. pebbles of refractory material heat the gas 15 from 300" to 750" F. ne pebbles are regenerated with

and good reflerability of the sorbents are at- Eributable to their improved physical characteristics.

hot air or aidsteam mixtures. Improved thermodynamic properties are indicative of The remaining U.S. patent references disclose various synergistic chemical effects in the mixed sorbents of the

regenerable H2S sorbents. Huff (15) discloses mixtures invention. The novel sorbents of the invention are char- of CuO with an oxide of a Group V or VI metal to 2o acterized by improved sorbent utilization, high HIS desdfurke a combustible gas at a temperature above removal efficiency and better sorbent stability. 200" C. in the presence of oxygen. Sugier (20) desulfiu- The sorbents of the invention are formed of porous, ized gases Containing HzS~ Cs29 Cos O r mercaptans high surface area particles. A first class of sorbents are

at temperiatura Of Ia0" co to 2oo" C- Group IIB or IB or VI11 metal oxides, deposited on a

'Om a form a mixture that melts within the temperature range

a salt .Or 30 usually from 500" C. to 700" C. The sorbent melts, coat-

H2S at 250" e. to 450" C. Martin et al(19) desulfurize a tion of the sulfidation period. During sulfidation simul- C02 containing waste gas with a regenerable carbon- taneous formation of a Group IB or IIB or VI11 sulfide sorbent containing a mixture of vanadium with K, Li or 35 Ba and optimally Al, Cr, si or Taylor et al (18) dis- and reduction of the Group VB or VIB to a lower metal close a porous solid catalytic element for oxide valent oxide may take place. As sulfidation pro-

and carbon automobfie gas. ne gresses the melt is gradually converted to a microcrys- catalytic talline mixture of the metal sulfide and oxide. The re- copper, or vana&W oxide interspersed with d-a 40 duced metal oxide chemiaorbs H2. Thus, both compo- and sodium oxide. nents of the coating absorb H2S which increases total

With several of the thermodynamically favorable, capacity. An example is the ZnO-V20!0s mixture. During high temperature H2S sorbents reported to date, slow sulfidation, zinc forms ZnS and v205 forms lower ox- rates of reaction and pore diffusion, sintering, and pore ides which chemisorb H2S. During regeneration by a plugging limit sorbent capacity and degree of purifica- 45 mixture of 0 2 and H20, the zinc component is con- tion under practical conditions. Such is the case with verted back to ZnO while the vanadium component to some commercial ZnO sorbents where reported con- V205 restoring the sorbent to its original molten state. versions at breakthrough were less than 20 percent. Restoring the melt after each regeneration prevents loss

Recent research has shifted from Pure to mixed metal 50 of surface area and absorptive capacity associated with oxides with the god of improving sorbent performance. unsupported solid ZnO sorbents. Fob exmPle9 zinc ferrite has been found to Possess The second class of high temperature sorbents con- better capacity and regenerability than pure zinc oxide sists of unsupported mixed oxide. fodg highly dis- (5)* Mixed zno-cuo has been studied for ks better persed solid solutions or solid compounds characterized resistance to surface area loss (24) and various other 55 by small crystallite high porosity and relatively

been studied with the objective of increasing the yield of Fe or Mo oxides.

ides form V ~ Q U S distinct cq~stallline phases or solid 6o Cu md possibly Zn or Mn also to remain in the oxidized compoundse and should generally possess different Eher- state which maximizes H2S absorption characteristics. modynamic properties and reactivity with respect to The sorbents are prepared in a special manner which reduction, sulfidation, and regeneration reactions. This

provides the sorbent in a highly porous form with a has so far received limited attention. 65 range of pore sizes. The prevents fouling of the sorbent

STATEMENT OF THE INVENTION during absorption and/or regeneration cycles. More efficient, regenerable H2S sorbents for high Both classes of sorbents exhibit high H2S removal

temperature use have been provided in accordance with efficiency, stable conversion, minimal pore plugging

~ t h &tUreS 0f vanadium oxide and iron oxide On 25 binary oxide mixtures such as Group VB or VIB with

M a t d a et d (21) sinter t i h W 9 molybdenum oxide high surface area refractory support. m e binary oxides

desulfurization sorbent. V O ~ et a~ (22) abs~rb sulfur of H ~ S removal, usually from 400" C. to 750" e., and One Of active metal

gases in Oxide Of a metal and then regenerated the 'Orbent With ing the pores ofthe support with a thin film at the incep-

can be at least one of manganese,

ZnO-Cr203 and Zno*A1203 have high surface areas. The sorbent is a mixture of Cu, Zn or

Of sulfur d ~ g regeneration Mked Ox- The presence of a preponderant of A1 allows Mn oxides with one or

5 and good regenerability. Synergistic effects and im- proved thermodynamics are displayed by the mixed oxides as compared to the pure oxides.

These and many other features and attendant advan- tages of the invention will become apparent as the in- vention becomes better understood by reference to the following detailed description when considered in con- junction with the accompanying drawings.

BRIEF DESCRIPTION OF THB DRAWINGS FIG. 1 is a schematic View of a system for producing,

cleaning, and utilizing a hot H2Scontaining coal gas; FIG. 2 is a set of breakthrough curves in successive

sulfidation cycles of the CAT-3N sorbent (1.7 weight percent ZnO, 3.6 weight percent V205, balance Al2O3).

FIG. 3 is a set of breakthrough curves in successive sulfidation cycles of the CAN-3N3 sorbent (1.6 weight percent ZnO, 4.8 weight percent V ~ 0 5 ~ balance A1203).

FIG. 4 is a set of breakthrough curves in sulfidation cycles of the CM-2 sorbent (5.1 weight percent CuO, 28.9 weight percent M003, balance Al203).

FIG. 5 is a set of breakthrough curves in successive sulfidation cycles of sorbent CFA (2 CuO.Fez03. Al- 203) at 650" C.

FIG. 6 is a set of breakthrough curves in successive sulfidation cycles of bulk CM-5 sorbent (CuMo04: Al- 203=1:1.1, molar) at 538' C. and 650' C.

DETAILED DESCRIPTION OF INVENTION The H2S sorber can be utilized in the system illus-

trated in FIG. 1. A hot gas is produced in coal gas generator 10 by the gasification of coal fed to the inlet 12. The coal gas generally contains from lo00 to 1O,o00 ppm H2. The coal gas at a temperature usually from 500" C. to 750" C. is fed through outlet line 14 to the H2S sorber 16 containing a bed 18 of porous sorbent. In one stage the sorber reduces the H2S content to less than 100 ppm, usually less than 5 ppm. The clean fuel gas is then fed through line 20 to the next unit 22 which can be a molten carbonate fuel cell, gas turbine genera- tor or a chemical synthesis plant.

When the bed 18 of sorbent is exhausted, valves 24 and 30 are closed and valves 26 and 28 are opened. A stream of regeneration gas containing oxidation agents such as oxygen and steam passes through the bed and strips sulfur from the metal sulfides and reoxidizes the metal ions to regenerate the sorbent.

The chemistry of a complete sulfidation-regeneration cycle may be represented by the overall reactions:

4,729,889

5

10

15

20

25

30

35

40

45

5 0

55 reduction

Hz,CO (1) Mox->Moy(~ < X)

sulfidation 60

MOy + YHZS = MSy + YHZO (2)

regeneration

MSy + M(SO& + MOx + XSOZ

where M is a metal selected from Fe, Cu, Zn, etc.

(3) 65

6 Reduction and sulfidation take place simultaneously

when the sorbent is contacted with the hot fuel gas. Regeneration can be conducted by air or an air-steam mixture where chemistry is more complicated than shown by reaction (3)) with elemental sulfur and H2S produced in addition to SOz

When reaction and diffusion rates are sufficiently rapid, the sorbent sulfur capacity and the extent of puri- fication are determined by thermodynamics alone. With many sorbents, however, the rate of reaction, pore dif- fusion, or diffusion in the product layer, limit sorbent capacity and degree of purification under practical con- ditions.

A. Supported Molten Mixed Oxide Sorbents In the first class of sorbents developed in this inven-

tion, a thin film of mixed metal oxide sorbents is coated onto the surface of a porous, high surface area refrac- tory support such as alumina or zirconia. The film be- comes molten under conditions of sulfidation. The ratio of the Group I or I1 metal to the Group V or VI can b e f 5 atomic percent from the eutectic point or slightly above. Representative eutectic forming films comprise V-Zn-0, V-Cu-0, Cu-Mo-0, Zn-Mo-0 or Fe-Mo-0. During sulfidation, the sorbent will be converted to a mixture of a well dispersed crystalline phase and a melt (perhaps incorporating some sulfide and dissolved H2S). By the end of sulfidation, much or all of the sor- bent will be in microcrystalline form, which will be brought back to the molten mixed oxide state upon oxidative regeneration.

The characteristics of this class of sorbents are large surface area and small crystallite size. These two prop- erties are expected to improve the kinetics of absorp- tion. In addition, synergistic effects may be present whereby the H2S removal efficiency of the sorbent is superior to the thermodynamic predictions for each of its pure solid oxide constituents.

Since formation of a thin film of molten oxide on the surface of the pores of the support during sulfidation is critical, the sorbent preparation technique is important. The metal oxide film is formed by dissolving water-sol- uble precursor salts containing the metals in the eutectic ratio in a minimum amount of concentrated ammonia hydroxide and distilled water. The compounds can be separately or simultaneously impregnated onto the sup- port. Co-impregnation is found to be the preferred tech- nique. The porous support is washed a few times in conc. ammonia hydroxide and dried slowly to 200" C. An approximately equal volume of the precursor solu- tion to the pore volume of the support is added to the support particles and the mixture stirred under vacuum. The mixture is then dried in air at elevated temperature usually 30" to 90" C. The impregnation-drying proce- dure is repeated until the loading of metal oxides on the support is from 10 to 60 weight percent usually from 20 to 40 weight percent. The support is then fully dried and calcined at a temperature above 300" C. in an oxi- dizing atmosphere to decompose the precursor salts and form a mixture of metal oxides.

7 4,'729,8 89

8 Some examples of mixtures of binary oxides which

form eutectics at temperatures below -700" C. are shown in Table 1:

TABLE 1 EUTECTIC COMPOSITION AND MELTING POINT

OF SELECTED BINARY OXIDE SY§TEMS

ACTIVE SORBENT MOLE RATIO FUSION, 'C. EUTECTICS

%MOflZ@i @) 0.98/1 641 CUOnZ@ @ 0.99/1 709 ~ 0 0 4 / % f & 3 w 0.44/1 707 cl&fO~.dM003 w 0.44/1 5 6

traces of NO3. Sorbents were prepared by either step- wise impregnation of Zn and V or co-impregnation.

The same sorbent impregnation technique described above was applied to all three supports to facilitate comparisons. The UCI-alumina was found to be more suitable than the MCB-alumina as a sorbent support became of its larger pore volume and average pore size. The zirconia support shown in Table 2 was used in a

10 few tests only. The properties of sorbents prepared with the UCI-alumina and zirconia supports are listed in Table 3.

SUPPORTED ZnO, V205 AND ZnO-V205 SORBENTS Method of Surface Area

Peooeertv ~umort Imoremation* Zn. Wt % V. Wt % (m2/szl ~~ ~~

CAT-3M UCI-alumina Co-impregnation 1.33 2.00 86 84 CAT-T-NZ from zinc ace- 1.69 -

tate 69 CAT-NZ2 from zinc ace- 3.20 -

tate (twice) CAT-NV from ammonium - 2.24 93

meta-vanadate (twice)

CAT-NV2 from ammonium - 1.88 93 meta-vanadate (twice) co-impeegnation 2.06 3.20 81 co-impregnation 1.26 2.70 84

$2 zirconia co-impregnation 0.388 0.60 23

O A l 1 impregnations were done in a conc. W O H solution.

CAT-3N2 CAT-3N3

See Table 2

FeMoOfloO3 (4 0.19/1 305 Experiments were conducted in a quartz microreac- tor system at approximately atmospheric pressure with

These sorbents exist as a melt coating the pores of a gas mixture containing 20 volume percent H2, 0.2-0.4 alumina or other high temperature substrates used for 35 volume Percent H2S, 0-7 volume Percent H20, balance catalysts. Properties of commercially available porous M2- The €W hourly space velocity was in the range of substrates are presented in Table 2: 2000-3000 hr-1.

TABLE 2 PHYSICAL PROPERTIES OF SUPPORT MATERIALS

fiO€=fiOY MCB-alumina u c I - al um ina ziconia source ALCOA F-1 United Catalysts, Inc. Alpha

T-2432 Products,

Bulk density Ob/ft3) Impurities (wt %)

Particle Size Surface Area (m2/g)

Poee Volume # (=&) Ave. Pore Sie # (A)

alumina

52

0.9 Na20, 0.08 Fe203, 0.09 SiO2, 6.5 H 2 0 (as loss on ignition) -30 + 40 mesh 240e 1W" 0.25-0.30

m

40

0.03 Na20, <0.01 each of of Fe203, SiO2, MgO, CaO, K20 and S. -35 + 45 mesh 85* 86.* 0.4

350

Inc. 31

2 AIz03, 0.1 Si02

-35 + 45 mesh 52O 250° 0.21

> 300

*As received. *'Treated by heating to - 850" C. for 2-3 hrs., thorough washing with a conc. N&OH solution and slow drying to 200' C.

Although the ave. pore size is 600.&, this alumina has a bimodal size distribution with -50% of W 5 A p a r a # Manufacturer's data

EXAMPEE 1.ZnQ-V2Q5 Supported Sorbents 60 The precursor compounds used for zinc md vana-

d i m are zinc acetate and ammonium metavanadate, respectively. Both decompose at about 200" C. to give the corresponding oxides, ZnQ and v 2 0 5 . In a few tests 65 zinc nitrate was used as a precursor. However, this decomposes at a higher temperature (400" C.) and re- quires further high temperature treatment to remove

The experimental data for sulfidation are presented in plots of outlet H2S concentration as a function of nor- malized absorption time th*, with t denoting real time and t* a calculated time corresponding to 100 percent ZnO conversion. Neglecting the vanadium contribution to H 2 S absorption, breakthrough conversions of sor- bents are calculated as moles of H2S retained/mole ZnO at the time when the outlet H 2 S concentration

4,729,889 9 10

starts to increase rapidly. Before breakthrough, the outlet H2S level was always less than 1 ppm.

The MCB-alumina, after stabilization at the reaction temperature (700' c.), had a surface area of - 100 m2/g. when this alumina Was impregnated With O d Y zno, a drastic reduction in surface area was noticed, suggest- ing pore blmkWe by z n o Crystallites. On subsequent impregnation with vzo5, the surface area again in-

tion a better dispersion of Zn is achieved. Tests with sorbents zn-v ratios Ievealed that around the eutectic composition the sorbent exhibits high HzS sorption capacity.

Zn0-Vz05

shown by measuring H2S elution in nitrogen purge following sulfidation. These results indicate a competi- tive HZO-H~S chemisorption mechanism.

As in the case of CATJN, the sorbent was also fully regenerable as shown by the breakthrough curves for cycles 2 and 5 in FIG. 3. T h w findings suggest that zinc is stabilized by vanadium in the alumina support, an important result from the standpoint of the overall per-

10 formance of ZnO-Vz05 sorbents. Moreover, no surface area 10s place during regeneration of these sor- bents, apparently because the molten VzO5.ZnO phase is restored at the end of each sulfidation/regeneration cycle.

Another finding from these tests refers to regenera- tion with steam. With steam and nitrogen mixtures (no

5

indicating that due to the Zn-V

A number Of tests were conducted with l5

supported On the MCB-alumina' with a propr sorbent Preparation, high breakthrough 'Orbent conversion (0'60-0.70) was Observed in the fmt

oxygen), regeneration was very slow and remained i,.,complete. With air and together (1-5 volume cycle. However, in all cases, in subsequent cycles the 2o percent 0 ~ ) ~ both H2S and SO:! were formed, and their conversion level rapidly dropped. From various obser- yields went through maxima at different times. Elemen- vations it was concluded that the support pore structure tal sulfur formation during wet regeneration was lower plays a key role in stabilizing the sorbent performance. (by about ten times) amounting to - 5 mol percent of It was found that the MCB-alumina has a bimodal pore

in pores than 50 A in size. This apparently regeneration performed at. the higher temperature of caused some segregation and pore blockge, and 750" C. eliminated zinc sulfate, which was formed to a thereby reduced accessibility of HzS to the unreacted limited extent at 700" c.

UCI alumina can suppress this problem. weight percent v205) was prepared by co-impregna- ln dry sulfidation at 700" c., the CAT-NZ (2.1 tion of the high surface area zirconia support described

weight percent zn0, no v205) sorbent showed cons& before. In sulfidation tests with S-2, the temperature erable loss of zinc by evaporation as did CAT-NZ2 used was 650" C. and the feed gas contained 20 volume which had a higher loading of ZnO. Tests with CAT- 35 Percent H2, 0.23 volume percent H ~ 2 . 3 volume percent W (4 weight percent V2O5, no ZnO) showed reten- H20, balance Nz. The first cycle resulted in a surpris- tion of 0.18 to 0.20 mol HzS/mol vzo5. N~ loss of ingly high breakthrough conversion of about 0.80. Fol- vzo5 was observed. However, Some soz was released lowing sulfidation, desorption and regeneration Steps during sulfidation, indicative of undesirable vZo5 re- Were camed Out. AS in the CaSe Of dry sulfidation (e.&, duction by HzS. 40 CAT3N, CAT-NV), the amount of HIS retained by

Tests with the CAT-3N sorbent containing the eutw- adsorption was about 20 percent of the total. This indi- tic ZnO/V2Os molar ratio of 51/49 show4 a stable cates that HzO/HzS ratios up to 10/1 might be allowed breakthrough conversion at about 0.65 over five cycles before a drastic inhibition of H2S chemisorption (by with a dry feed gas containing 4OOO ppm H2S. FIG. 2 45 H2O) ~ U X E L shows the corresponding HzS breakthrough curves. In the second and following cycles a drop in the This high and stable conversion indicates rapid kinetics, ' absorption Capacity of the s-2 Sorbent was observed, no metal loss, and little or no loss of surface area in probably due to a pre-reduction step with a gas successive cycles. mixture. This pre-reduction was used in the beginning

Regeneration of this sorbent was conducted with 1 50 of the second cycle, upon which zinc metal could have volume percent 02 in Nz (no H20) at 700" C. Sulfur been lost and the composition of the sorbent changed. products included H2S, SO2 and elemental sulfur. Pre- The zirconia support used in the S-2 sorbent prepara- ceding the oxidative regeneration step, a nitrogen purge tion may also have contributed to the observed instabil- was used (also at 700" C.), upon which H2S and elemen- 55 ity of this sorbent. tal sulfur were produced. The total amount of elemental The main features of the ZnO-VzOs sorbents can be sulfur in the sulfur products was as high as 50-65 vol- summarized as follows: ume percent. (1) In each sulfidation the initially existing molten

In a series of experiments with the vanadium-rich sorbent phase disperses ZnO, VzOx, etc., better in the Z ~ O - V Z O ~ CAT-3N3 sorbent, the effects of steam ad&- 60 support. As a result, reproducible high breakthrough tion in sulfidation and regeneration were examined. In conversions (dry: 60-70 percent; w/steam: 10-40 per- dry sulfidation with 2000 ppm H2, CAT-3N3 consis- cent) are obtained. tently gave breakthrough conversions of 0.64. How- (2) Vanadium stabilizes ZnO in the alumina support. ever, as shown in FIG. 3, in the presence of H20 (6.7 65 (3) HIS is chemisorbed up to a level of 20 mol percent volume percent), the breakthrough conversion was on V20, (no measurable sulfide formation occurs). At only 0.15. In this case, the total H2S retained by chemi- high H20/H2S ratios, the HzS uptake by vanadium is sorption on the sorbent was greatly reduced. This was minimal apparently due to preferred adsorption of H20.

structure with most of its surface area a d pore volume 25 the sulfur products' it was later found that

zinc. A more open pore structure as provided by the 3o s-2 (with Oa5 weight Percent zno and l.l

4,1929,889 88

(4) Regeneration at high temperatures (up to 750" C.) is feasible without any loss of surface area (sintering) or active metal loss, and eliminates the formation of zinc sulfate.

(5 ) Dry regeneration produces about ten times more sulfur, and an important step in elemental sulfur forma- tion is the catalytic decomposition of H2.

(6) Since vanadium oxides do not appreciably con- tribute to H2S sorption, the overdl sulfur capacity of ZnO-V2O5 sorbents is low. It may be increased if V2Q5 is replaced by a reactive compound.

EXAMBLE 2. (Zn,Cu)-Mo-O Supported Sorbents In view of the hitations of the ZnO-VzOs Systems it

was decided to explore the zinc and copper molybdates eutectics with Moo3 melting, respectively, at 705" C. and 560" C. (10). Moreover, molybdenum is also known to react with H2S to form sulfides (mainly Mo2S3). Hemce, the overall capacity was expected to be higher with these systems than with the Zn-\I sorbents.

In a first experiment a eutectic melt was prepared by mixing powders of zinc molybdate (ZnMoO4) and mo- lybdenum oxide (MoQ3). After cooling, the solidified crystalline materid was crushed to obtain a powder. This powder was mixed with the high sufce area UCL alumha and tested for its H2S absorptiom capacity. With a sulfidation gas containing 20 mol percent H2, 1 mol percent H2, 26 mol percent H20, balance N2, a 1-15 percent breakthrough conversion (based on Zn and Mo) was obtaimed at test temperatures of 538" and 720" C. This was an encouraging result, given the fact that the eutectic melt was basically unsupported in this sorbent formulation.

In parallel work with porous, mixed oxide sorbents, it was discovered that copper oxide, which by itself is thermodynamically inferior to zinc oxide as a H2S sor- bent shows superior behavior when combined with certain other oxides, as in copper ferrite. This, in con- junction with the fact that the eutectic in the Cu-Mo-0 system is formed at a much lower temperature (560" C.) prompted testing of a supported Cu-Mo sorbent. This would also eliminate the sulfate formation problem associated with zinc-based sorbents because of the lower thermal stability of copper sulfate. Sorbent CM-2 was prepared by co-impregnation of copper carbonate and ammonium molybdate on alumina by an incipient wetness technique to obtain -34 weight percent total loading. The amounts of CuO and Moo3 in CM-2 were in the desired proportion (1 mol CuO: 3.125 mol MoO3) for eutectic formation. The surface area of the fresh sorbent was - 53 mVg. X-ray diffraction 0) analy- sis of CM-2 identified CuMo04 and A1203 in the crystal- line phase along with some poorly crystdine material. In tests with the CM-2 sorbent conducted both at 538" and 650" C. with H20/ H2S=25/P9 molar and H20 content as high as 25 mol percent, the sorbent conver- sion (based on both Cu and Mo) at breakthrough was - 25 percemt, FIG. 4. A more significant result was that the outlet H2S level was less than 0.1 ppm until break- through. Regeneration was carried out at 650" C. with a 70 mol percent N2-30 mol percent air mixture. The

A1203 in the crystalline phase. No evidence of molybde- num sulfides was found. Apparently, reduction of MOO3 not followed by sulfidation takes place during reaction. Calculating the sorbent conversion in terms of Cu alone gives better than 90 percent conversion at breakthrough.

W e no molybdenum sulfides are formed, the high H2S removal eficiency of this sorbent is attributed to

10 the presence of Mo as well as Cu, since a complex cop- per molybdate (and perhaps sulfide) eutectic formed during sulfidation can retain additional amounts of H2S. This explanation is offered for the sub-equilibrium H2S breakthrough levels observed with CM-2 as well as with bulk Cu-Mo-based sorbents.

During sulfidation of the CM-2 sorbent, limited evap- oration of molybdenum was observed. The molybde- num loss (measured by atomic absorption (AA) analy-

20 sis) was higher at 650" C. At 538" C., the loss of molyb- denum was low, however, it was not completely elimi- nated. Tests with other Cu-Mo-based sorbents indicate that the molybdenum loss may be suppressed by using a

25 different (copper-rich) sorbent composition which also increases the overall sulfur loading of the sorbent.

5

B. Unsupported Oxide Solid Solutions The second class of high temperature sorbents ac-

30 cording to the present invention consists of unsupported mixed oxides in the form of solid solutions or solid compounds. One instance of a successful mixed oxide system is the stoichiometric zinc ferrite spinel, which was found (5) to have properties superior to those of

35 both iron and zinc oxides. It may not be necessary, however, to use a compound with definite stoichiome- try and crystal structure. Thus, the second class of sor- bents involves stoichiometric or non-stoichiometric

40 mixed metal oxides [Zn-FerOy, Cu-FerOy, Cu-(AI,- Fe)rOy9 CU(MO,A~)~O,, etc.] prepared by special tech- niques as highly dispersed, microcrystalline solids. Their main attributes are high surface area (10-100 m2/g) and large pore volume (1-3 cc/g) in unsupported form which are expected to provide rapid absorption and regeneration rates. While the thermodynamic prop- erties of these solid solutions have not been determined, it is possible that they are superior to those of the pure

50 components. Since these sorbents do not use an inert support, they should exhibit high capacity on a unit weight basis.

Several methods are described in the literature 55 (1 1-14) for synthesizing highly dispersed mixed oxides.

Early attempts to form high surface area metal oxide sorbents by precipitating mixed carbonates from ho- mogenous salt solutions followed by drying and calcin- ing resulted in materials of low surface area, about 5-8

60 m2/g. The bulk sorbents either in single or mixed form were then prepared by a technique that resulted in high pore volume and surface area. Following a general procedure suggested in the literature (14), an aqueous

65 solution of thermally decomposable metal salts in the desired proportion and an organic polyfunctional acid

45

performance was stable in two cycles. XRD analysis of the sulfided sorbent identified cU1.96S9 Moo2 and

containing at least one hydroxy andat least one carbox- ylic function is rapidly dehydrated under vacuum at a

13 4,729,889

14 temperature below 100' C. An amorphous solid foam forms which is calcined at an elevated temperature above 300' C., usually 500' C. to 600' C., to form a

bent had a surface area of 26 m2/g and consisted of CuO, FeA1204, CuFe204, and A1203 phases. Properties of these sorbents are found in the following table:

TABLE 4 CRYSTALLINE PHASES AND SURFACE AREA OF VARIOUS SORBENTS

BET SURFACE

COMPOSITION AREA

CuO:Mo03:A120j N/A 18.2 SORBENT (molar ratio) PHASES (XRD) (m2/g) CM-3, fresh

CM-5, fresh

CM-5, sulfided

ZF, fresh

ZF, sulfided (538' C., 6 cycles) ZF, regenerated (700' C., 6 cycles) CF, fresh

CF, sulfided (538' C., 8 cycles) CF, sulfided (650' C., 2 cycles) CF, regenerated

CA, fresh

CA, sulfided

CA, regenerated

CFA, fresh

CFA, sulfided (650' C..' 2 cvcles)

(700' c. , 2 cycles)

(538' c . , 5 cycles)

(700' c. , 4 cycles)

1:3.125:26.6 CuOMo03:A120j Cu3Mo209, amorphous 1:l:l.l material

MO02, c~004, cU1.96s, A12O3, CuAIS,

ZnOFe2Oj ZnFe204 1:l

P-ZnS, Fel,S

ZnFe204, Fe203, amorphous material (probably ZnO)

CuO.Fe203 CuFe204 1:l

CuFe204, Fe304, CuFeS2, Fel.3, amorphous material Fe3O4, CuFqOq, Fel-3, amorphous material Fe2O3, CuO, CuFe204, amorphous material cuA1204, c u o

CuA1204, Cu7S4, amorphous material CuO, CuAl2O4, amorphous material

CuO:A1203 I:1

CuOFe203:A1203 CuO, CuFe204, FeA1204, 2:l:l amorphous material

CuS, FeS, CuFe2S3, amorohous material

6.2

N/A

17.0

3.0

6.0

7 .o

1.2

1 .o

2.0

12.0

4.2

N/A

26.0

N/A

mixed oxide phase. The crystallized mixed oxides thus ne sorbents were loaded in a quartz microreactor (1 formed are homogeneous and highly porous. cm I.D.) as -20+ -40 mesh particles mixed with low

The sorbent material Prepared by the latter direct surface area zirconia or alumina particles, which served calcination technique has low density, a surface area as an inert filler. The use of the inert particles allows above l5 m2/g and POre exceeding la5 cc/g* compression of the experimental run time by decreasing scanning electron micrographs indicate the presence Of the capacity, while maintaining the Same space large po re 1-20 pm in diameter and low angle XRD velocity. revealed that the solid was microcrystalline with aver- ne experiments consisted of alternating sulfidation age crystallite size of about 500-1000 Angstroms. 45 and regeneration runs. In a sulfidation run the sulfur-

The porous mixed oxides contain one oxide of Group free sorbent was exposed to a feed gas containing H2 IB, IIB or VIIA metal such as copper, Zinc Or mmanese (15-20 percent), H20 (7-25 percent), HzS (0.2-1 per- and One Or more Oxides Of a Group I1IA9 cent) and N2 (balance). The temperature was chosen in metal such as aluminum, iron or molybdenum. The 5o the range 500" -700" C. and was held fixed for the dura- Presence Of aluminum Or Uon Oxides has an effect On the tion of sulfidation. Sulfided sorbents were regenerated oxidation state allowing the Group IB, IIB or VIIA using a nitrogen-air or a stem-air mixture at tempera- metal to remain in the oxidized state during sulfidation. tures of 600" -700" c. Molybdenum addition in preselected amounts results in

ZnFezO4 Sorbents the formation of a eutectic at sulfidation temperature 55 and improves the efficiency of the sorbents. Zinc ferrite is one of the most efficient H2S sorbents.

The above described method was used to prepare the Extensive lab and bench-scale tests with ZnFezO4 have bulk sorbents CF (CUFQO~), Z F (ZnFezO4), CA (CuA1 been performed at DOE/METC (5). The material used ), CFA (Cu-Fe-Ala), and CM (Cu-Mo-Al-0). All in that work was prepared by United Catalysts, Inc., by these materials were prepared as highly dispersed mi- the conventional technique of high-temperature (> 800" crocrystalline solids with very large (Z 10 pm) as well C.) heating of mixtures of the pure oxide powders. The as submicron size pores. The gross porous structure surface area of this zinc ferrite, designated METC-ZF, before and after sulfidation appears very similar. X-ray is - 5 m2/g and its pore volume -0.3 cc/g. diffraction analysis showed that the mixed oxides ZnO- 65 The performance of porous zinc ferrite (ZF-C) syn- Fe2O3, and CuO-FezO3 had the ferrite spinel structure, thesized in this invention having high surface area and ZnFezO4 and CuFezO4, respectively, while CuO-Al203 large pore size was tested in the quartz microreactor formed the aluminate spinel CuAl2O4. The CFA sor- and compared to the METC-ZF sorbent at the same

Of

4.729.8 89 15

operating conditions. Both sorbents were used as -20+40 mesh particles mixed with low surface area alumina particles.

At 538" C. (1OOO" F.) sulfidation temperature, the stable pre-breakthrough conversion of ZF-C was >0.75, while that of METC-ZF was 0.35-0.40. Both sorbents were capable of removing H2S from -2500 ppm to less than 1 ppm level with a feed gas containing 20 volume percent Hz and 6.5 volume percent H20. The higher conversion of ZF-C must be attributed to its different physical properties. SEM micrographs of the sulfided ZF-C and METC-ZF sorbents show that the pore structure of the two is very different with METC- Z F lacking the large pores of ZF-C. The gross morpho- logical features of the latter were the same before and after sulfidation which is indicative of high accessibility to HzS and limited or no pore mouth blocking.

Similarly high conversion (>0.75) was obtained with ZF-C at all temperatures 538" C. -650" C. and with the sulfidation gas containing 15-20 percent Hzp 7-25 per- cent HzOs 0.2-1 percent HIS, balance Nz by volume. In sdfidation tests at temperatures > 400" C., however, all zhc ferrite sorbents (ZF-C, MEW-ZF) began to lose zinc (via reduction of ZnO). In deposits collected kom the cooler part of the reactor tube, zinc was identified by Aton~k Absorption (Mi) d y ~ k . LOSS of zinc could not be prevented even with high P I 2 0 and low H2 concentrations in the feed gas.

At 600' C., with 15 percent Hzs 25 percent H20, 1 percent H2S, 59 percent N2 by volume in the feed gas, the rate of evaporative Zn loss was 8 and 26 weight percent per lo00 hours at space velocities of 2100 and 6900 h r l , respectively. A slow decline in conversion

16 - ,- - -

version, sub-equilibrium H2S levels were measured, again indicative of chemisorption effects.

Characterization of fresh, sulfided and regenerated samples of CF-sorbents was performed by SEM and XRD analyses. The gross porous structure of the sor- bent before and after sulfidation appeared very similar in SEM (as in the case of zinc ferrite). XlRD analysis of the sorbent sulfided at 538" C. identified the mixed sul-

10 fide compound CuFeSz (chalcopyrite) along with Fel. xS, Fe30h unconverted cd?e04 and some amorphous material. In the sample sulfided at 650" C., the phase CuFeS2 was absent, while there was indication of a poorly crystalline Cu2S phase.

These results suggest that at 538-600' C. sulfidation temperatures the reduction of CuO is suppressed in the ferrite-spinel. The presence of CuFeS2 in the sample sulfided at 538" C. along with the observed HzS pre-

20 breakthrough levels correspond to the sulfidation reac- tion:

5

25 The intermediate compound CuFe02 indicates a step- wise reduction of CuFezO4. The equilibrium H2S levels for the above reaction were calculated by using themo- dynamic values for the compound Cu20.Fe203 (25).

30 These are in agreement with the experimental HIS lev- els, and much below the values for metallic copper sulfidation. At the higher sulfidation temperature of 650" C., reduction to metallic copper is very fast, and controls the pre-breakthrough levels of H2S after an

35 initial sorbent conversion (0.15-0.20) at sub-equilibrium H2S exit levels.

was observed between the first and fourth cycles of sd fidation/regeneration.

The stability of porous copper aluminate (CA) of the CuFe284 Sorbents 4o spinel crystal structure was investigated in a series of

sdfidatiodregeneration tests at 538"-650" c. The re- que described earlier* H2S breakthrough curves in sul ts were very S i m i l a r to copper ferrite. Thus at several sulfidation cycles at 538" C. showed that the 538O-W c.9 the H2s Pre-breakthroUgh levels were pre-bre&through sorbent conversion was high (-0.80) 45 below the equilibrium for copper sulfidation reaction and stable (over 6 cycles). In all cases the pre-break- (I), and high (>0-75, based on CWS) and stable (over through H2S level was below the value of 44.5 ppm 5-6 cycles) sorbent conversion was observed. Copper in calculated for the equilibrium of metallic copper sulfi- the +2 or + 1 oxidation state was apparently stabilized

in the alumina matrix and controlled the exit level of PIIS till breakthrough. At 650" C., the HIS exit levels

dation.

2Cu+H2S-*CuzS+HzO (1) after -0.20 sorbent conversion corresponded to sulfi- dation of metallic copper. With copper aluminate, how-

In fact, the HZS level rmxihed below 2 PPm until 0.5 ever, structural changes that occurred at 650" C. were conversion and then gradually increased to - 10 ppm 55 reversed when the temperature was lowered to 538" C. when the conversion reached 0.78. One possible expla- nation for such sub-equilibrium concentrations is H2S

Mixed Cu-Fe-Al-0 (CFA) Sorbents chemisorption to form a surface sulfide layer with sub- stantidy lower free energy than the bulk sulfideo In view of the enhanced stabilization of copper oxide

In two sulfidation-regeneration cycles carried out at in copper aluminate sorbents, a mixed copper ferrite- 600" C., sub-equilibrium H2S levels were again mea- copper aluminate material, CFA, with 2CuO:Fe203:Al- swed till breakthrough, which occurred at -0.80 sor- 2 0 3 molar ratio was prepared in porous bulk form (with bent conversion. At 650" C. sulfidation temperature, 26 m2/g surface area) as a potential sorbent for higher however, while a skdarly high and stable sorbent con- 65 (> 600" C.) temperature H2S removal. Under similar version took place, the pre-breakthrough HzS level at operating conditions, this sorbent was superior to either 0.80 sorbent conversion was 85-90 ppm, close to the copper ferrite or copper aluminate in regard to pre- equilibrium for reaction (1). Up to -0.20 sorbent con- breakthrough H2S levels, which were lower than the

CuA12O4 Sorbents

porous copper fe&e (CF) was prepared by the

50

was not true with copper ferrite sorbents.

4.729.889 17

equilibrium of copper sulfidation reaction (1) at all tem- peratures, 538" C.-650' C. An example is shown in FIG. 5 for 650' C. sulfidation temperature. XRD analysis of the fresh and sulfided (after cycle 4) CFA sorbent was performed to identify stable crystalline phases that may be responsible for the improved performance of this sorbent. The fresh CFA consisted of CuFe204, CuO, FeAl204 and some amorphous material (probably Al- 203), with FeA1204 (iron aluminate) in highly micro- crystalline form. The sulfided sample contained CuS, FeS and CuFezS3 along with amorphous (to XRD) alumina. These results are very important, indicative of stabilization of both Cu2+and Fez+in the sorbent ma- trix and associated improved absorption equilibria.

Additional testing of the stability and performance of the CFA sorbent was conducted at the higher tempera- ture of 830' C. with a reactant mixture containing 30 volume percent H2, 17 volume percent H20, 1 volume percent H2S, balance N2, simulating the Texaco gasifi- er-fuel gas. Breakthrough of H2S took place at complete (100 percent) sorbent conversion, while the pre-break- through H2S level was zero up to -0.20 sorbent con- version and -270 ppm until final breakthrough.

A summary of sulfidation tests with the ZF, CF, CA and CFA sorbents is presented in the following table:

18 loss of a molybdenum compound. Other preparations of Cu-Mo-AI-0 sorbents were then made in porous bulk form with progressively lower Mo-content to eliminate the molybdenum-loss problem. All sorbents were capa- ble of removing H2S to <0.5 ppm level and high sor- bent conversions (0.5-1.20 based on copper) at break- through were typically observed.

Sorbent CM-3 with a molar ratio of CuO:Mo03:Al- 10 203=1:3.125:26.6, was prepared in dispersed form ac-

cording to the technique described previously. The aluminum component was introduced in the precursor salt solution in the form of aluminum nitrate.

Sulfidation tests with CM-3 were conducted at 538" C. with inlet gas molar composition of 20 percent H2,25 percent H20,54 percent N2, and 1 percent H2S. Regen- eration was carried out with a 70 percent N2-30 percent air mixture. The H2S breakthrough curves in two cycles

20 show pre-breakthrough through H2S levels nearly zero as achieved with the supported CM-2 sorbents. The sorbent conversion (based on Cu alone) was quite high (75 to 80 percent).

At the end of each sulfidation the appearance of the 25 sorbent through the quartz reactor walls indicated the

formation of a molten phase during reaction which could have retained additional amounts of H2S. As with

TABLE 5 SULFIDATION TESTS WITH SORBENTS ZF, CF, CA AND CFA

SULFUR PRE-BREAK- BREAKTHROUGH LOADING

FEED GAS THROUGH SORBENT T, COMPOSITION HIS CONVERSION+ ( Es )

SORBENT CC.) (mol %) (PP& (t/t*) \ g sorbent )

- ZF 600 15 H2, 25 H20, 3 Zinc 1 HzS, 59 Nz Ferrite

CF Bulk 538 20 H2,6.8 H2O 5-10 copper 0.26 H2S, 72.94 N2 Ferrite 650 90

600 15 H2.25 H20 38

-

1 H2S, 59 N2

CA Bulk 538 20 H2,25 H20 < 10 Copper 1 H2S, 54 Nz Aluminate 650 100

CFA Mixed 538 20 Hz, 25 Hz0 < O S Copper 1 HIS, 54 N2 Ferrite- 600 I5 Hz,25 H20 25

-

-

Copper Aluminate 0.5 HzS, 59.5 N2

650 50 830 30 H2, 17 H20 250

0.808 0.318

0.80

0.90 0.80

0.75

0.80

0.60

0.60 .

0.267

0.300 0.267

0.066

0.070

0.137

0.137

0.65 0.148 1.00 0.228

'Based on the active sorbat components. *The ratio t/t* showed a gradual decrease in successive cycles.

Mixed Cu-Mo-Al-0 (CM) Sorbents 60 Previously described sulfidation tests with CM-2, an

alumina-supported mixture of CuMoOr:MoO3 (=0.47/1 molar) which forms a eutectic at 560" C. (lo), showed that this material was capable of complete H2S 65 removal (to less than 0.5 ppm level) in the temperature range of 538'- 650" C., under typical sulfidation condi- tions. The only limitation appeared to be evaporative

the CM-2 supported sorbent, limited loss of molybde- num was observed during sulfidation of CM-3. This prompted the preparation of the CM-5 sorbent, de- scribed below, with lower Moo3 content.

CM-5 sorbent was prepared in dispersed form with a molar ratio of CuMo04/A1203= lA.1, as a potential improvement over sorbent CM-3 (to eliminate the mo-

19 4,729,889

20 lybdenum loss). XRD analysis of the fresh sorbent iden- tified Cu3Mo209 in the crystalline phase, while the remaining Moo3 and the A1203 were in a phase amor- phous to XRD (Table 4).

Sdfi&tion tests with CM-5 were conducted at 538" c. and 6500 c. me fiet gas molar composition was 15 percent H29 59.5 percent 1\12, 25 percent H20 and 0.5 percent H ~ S ~ Regeneration was carried out w i ~ a 90 percent ~ ~ - 1 0 the be-g of regeneration was 538" c., then was raised to 650" C.

the bed indicating that loss of molybdenum had taken place from this sorbent at 650" C.

The sorbents of this invention have shown improve- ments (higher sulfur capacity, limited surface area loss, high stability and good regenerability) that can be at- tributed to their improved physical characteristics. In addition9 improved thermodynamic sorbent Properties have been identified indicative of synergistic chemical

Supported Zn-V-0, CU-Mo-0 sorbents prepared ac- cording to the invention by impregnation of high sur-

-me. n e temperame at 10 effects in the mixed oxide sorbent formulations.

The &S bre&kough curves obtained for sorbent CM-5 are presented in FIG. 6. As can be s e n from this

face area alUmh'la weight percent show the following advantages:

m2/d to a loading of 5-3O

Enhancement of reaction rates due to the initial mol- ten phase and subsequent high dispersion of the crystal- line phases formed.

Very high H2S removal efficiency (> 99.99 percent), 20 which translates to pre-breakthrough H2S levels of

figure, the H2S level in cycle l was close to zero till nearly complete sorbent conversion based on Cu alone. The sorbent continued to be active even after complete theoretical conversion of copper, possibly due to mo- Iybdenum contribution. The final H2S breakthrough at - 10 ppm took place at a sorbent conversion of about 1.20 (based on copper). Direct observation of the sor- bent through the quartz reactor wall again indicated the formation of a molten phase during sulfidation. Perhaps a complex sulfide containing eutectic mixture of copper molybdates is formed during sulfidation. As discussed above ~ O K CM-3, this phase may explain the sub-equilib- rim M2S levels observed throughout the sulfidation of Cu-Mo based sorbents.

The second sulfidation cycle at 538" C., FIG. 6, gave a slightly lower pre-breakthrough sorbent conversion (- 1.15 based on copper) and - 20 ppm H2S level. This behavior, typical of all unsupported sorbents, can be attributed to limited sintering (loss of surface area) and structural rearrangement particularly during the higher temperature (650" C.) regeneration conditions. The interesting fmding from both sulfidation-regeneration cycles with CM-5 is that no molybdenum loss took place. Thus, the CM-5 composition provided the neces- sary improvement over CM-3, which contained excess Mo83.

In a third cycle with CMS, the sulfidation tempera- ture was raised to 650" C. The corresponding H2S breakthrough curve, shown in FIG. 6, has some inter- esting characteristics. First, zero H2S breakthrough up to -0.40 sorbent conversion was observed. Thereafter, the H2S level increased slowly to -90 ppm at sorbent conversion of 0.70 (based on copper), and to - 110 ppm before the final (third) breakthrough at sorbent conver- sion of 1.20. The three segments of the breakthrough curve of FIG. 6 are indicative of different sulfidation mechanisms, the fmt being the most important with zero H2S level.

<0.5 ppm.

than for each oxide alone. Synergistic effects-better thermodynamic equilibria

Stabilization of ZnO (no zinc loss). Limited molybdenum loss from Cu-Mo sorbents at

>W C. may be avoided by careful choice of Cu/Mo ratio in the sorbent.

Fully regenerable sorbents; no loss of surface area. No sulfate formation in regeneration (at 700" c). Mixed metal oxide sorbents prepared via pyrolysis of

homogenous organic precursors as highly dispersed solids (SA> 10 rnz/g, pore volume -2.0 cc/g) show the following improvement over conventional sintered ma-

(a) Rapid sulfidation rates; sharp H2S elution profiles. (b) High (>70 percent) and stable breakthrough sor-

30

35 terials:

bent conversion. 4o (c) No pore plugging.

(d) Complete regeneration. A comparative evaluation of the performance of cer-

tain porous mixed oxide sorbents is given below in terms of sulfidation temperatures (for 25 volume per-

45 cent H20, 20 volume percent H2, 1 volume percent H2S, balance 1\12 in the feed gas).

At 538" C. (1OOO" F.), the porous ZnFe204, Cu- Fe-Al-0 (CFA) and Cu-Mo-Al-0 (CM) sorbents are all very efficient for H2S removal from 0.5-1.0 volume

50 percent in the feed gas to 1-5 ppm breakthrough levels (at 70-75 percent sorbent breakthrough conversion). With CuO-containing sorbents, reduction to metallic copper is suppressed as indicated by the pre-break-

55 through H2S levels, which are well below those corre- sponding to metallic copper sulfidation. Intermediate compounds retaining copper in the f 2 oxidation state are inferred from the data.

A sample of the CM-5 sorbent sulfded at 650" c. Was At 600" C. (1 112" F.), zse204, while still very effi- analyzed by XRD "he m a t e d had hi& C W e W 60 cient for H2S removal, shows zinc metal loss in the and contained some CuxS, little unconverted CuhaOQ4, vapor amounting to at least 8 weight percent Zn/lOOO and A1203 cable 4). Its major components were Moo2 hours even with reduced (15 volume percent) H2 con- and an unidentified complex phase of CuAlS, contain- tent in the feed gas. ing some Mo). Further characterization is clearly neces- 65 CuFe204, CFA at the same conditions show very sary to elucidate the sulfidation mechanism in this com- high (>60 percent) and stable sorbent conversion at 35 plex sorbent. At the end of sulfidation, a thin deposit ppm H2S breakthrough, which is lower than the metal- layer was observed on the reactor wall downstream of lic copper sulfidation equilibrium.

4,729,889 21 22

CM-sorbents exhibit very high HIS removal eff- ciency (to <0.5 ppm level) that may be due to the for- mation of complex sulfide and oxide eutectics with high absorptive capacity.

At 650' c. (1200" F-1, the Operating temperawe of molten carbonate fuel cells, zflQo4 Was excluded from use because of much higher 2% metal losses at this temperature.

bent conversion at -85 PPm HIS breakthough level* which now corresponds to metallic copper sulfidation equilibrium. Evidently, at this temperature reduction of Cuo-tcu precedes sulfidation.

mixture of a first Group VB or VIB metal oxide and a second Group IB, IIB or VI11 metal oxide, said oxides being present in the mixture such that the ratio of the metal oxide of the fmt group to the metal oxide of the second group is within +5 percent of the atomic ratio of the metal oxide of the first group to the metal oxide of the second group at the eutectic point and said layer being molten at a temperature from 400' C. to 750" C.

2. A cornposition according to claim 1 in which the refractory support is selected from alumina or zirconia.

3. A compos~t~on according to claim 2 in which the binary oxide mixture layer is present on the support in an amount from 5 to 60 percent by weight.

caeZ04 shows high (>a percent) and stable sor- 10

cFA (Cu-Fe-Al-o) shows high (>75 percent) and l5 4. A composition according to claim 2 in which the binary metal oxides mixtures are selected from V-Zn-O, sorbent conversion with prebreakthrough H2S

levels in the range of 0-10 ppm up to 0.50 sorbent con- v-cu-o, Cu-Mo-O, Zn-Mo-O or Fe-Mo-O. version and less than -35 ppm at breakthrough. 5. A composition according to claim 4 in which bi-

nary metal oxides mixtures are selected from ZnO/V- sorbent is considered to be the most attractive for H2S 205, c u o / v ~ o ~ , ~ ~ M o O ~ o 0 ~ , CuMo04/MoO3 or removal at 650' C. or higher temperatures. Because of its alumina content, this sorbent may be expected to also FeMoOflo03.

6. A composition according to claim 2 in which more possess better attrition-resistance (mechanical strength) 25 than 50 percent of the pores in the refractory support in practical applications.

C u - ~ e - ~ - O (CFA) or cu-hiO-~-o (CM) sorbents much higher H ~ S removal efficiency 7. A composition of matter consisting essentially of

alone or CFA h e d with c~ in a single stage are 3o persed microcrystalline solids of mixed oxides of a cop- projected = very efficient for H ~ S removal at per or manganese oxide with at least one oxide selected 650" C. for molten carbonate fuel cells or gas turbine from an al-num, iron, or molybdenum oxide, said applications. The sorbents of the invention will also find Particles having a surface area from 15 to 100 m2/g and use in SO2 removal from combustion gases or smelter a Pore volume from 1-3 CC/g. gases. 8. A composition according to claim 7 in which the

It is to be realized that only preferred embodiments of porous, mixed oxides are selected from Cu-Fe-0, Cu- the invention have been described and that numerous (Al,Fe)-O or Cu(Mo,Al)-O. substitutions, modifications and alterations are permissi- 9. A composition according to claim 7 in which the ble without departing from the spirit and scope of the 4o mixed oxide particles contain pores having a diameter invention as defined in the following claims. ranging from 1 to 20.

10. A composition according to claim 7 in which the mixed oxides are selected from CuFezO4, CuA1204, Cu-Fe-Al-0 and Cu-Mo-Al-0.

From this evaluation, therefore, the porous CFA 20

have a diameter exceeding about 100 Angstroms.

than each of the pure constituents alone. Either CFA poroUs9 unsupported particles including highly dis-

35

We claim: 1. A composition of matter in the form of porous

particles consisting essentially of a refractory metal oxide support containing a layer of a binary metal oxide 45 * * * * *

50

55

65

UNlTED STATES PATENT AND TRADEMARK OFFICE

CERTIFICATE OF CORRECTION PATENT NO. : 4 , 729 , 889 DATED March 8, 1988

INVENTOR(S) : Maria Fly tzan i -S tephanopoulos e t a1

are hereby corrected as shown below: Item [19] and Inventors [75] correct "Flytzani"

It i s certified that error appears in the above-identified patent and that said Letters Patent

A b s t r a c t (57 , l i n e 1 2 , change Itof I' , second occurrence , t o

Column 2 , l i n e 38, o m i t "deteimined" Column 2 , l i n e 68, change ' ' sufur" t o --sulfur-- Column 4 , l i n e 15 , correct s p e l l i n g of " r e g e n e r a b i l i t y " Column 4 , l i n e s 27-28 , a f t e r f fox ides" , add --must-- Column 4 , l i n e 4 0 , correct s p e l l i n g of "chemisorb"

Column 4 , l i n e 6 5 , change "The" t o --This--

Column 8, Table 3 , i n the subheading a f t e r "Suppor t" , add

Column 8 , Table 3 , i n t h e penul t imate l i n e , b e f o r e "See"

Column 1 3 , l i n e 4 7 , correct s p e l l i n g of "manganese" Column 13, l i n e s 58-59 , change If ( C u A 1 ) t o -- ( C u A 1 2 0 4 ) -- Column 1 4 , l i n e 38, change ' I-20+-40" t o -- -20+40- -

--or--

+ -- -- add -- + --

Signed and Sealed this

Ninth Day of August, 1988

Attest:

DONALD J. QUIGG

Attesting Officer Commissioner of Patents and Trademarks